import os

from flask import Flask


def create_app(test_config=None):
    # create and configure the app
    #instance_relative_config = True告诉应用配置文件是相对于instance folder的相对路径。
    # 实例文件夹在flaskr包的外面，用于存放本地数据（例如配置密钥和数据库），不应当提交到版本控制系统。
    app = Flask(__name__, instance_relative_config=True)

    app.config.from_mapping(
        SECRET_KEY='dev',
        DATABASE=os.path.join(app.instance_path, 'flaskr.sqlite'),
    )

    if test_config is None:
        # load the instance config, if it exists, when not testing
        app.config.from_pyfile('config.py', silent=True)
    else:
        # load the test config if passed in
        app.config.from_mapping(test_config)

    # ensure the instance folder exists
    try:
        os.makedirs(app.instance_path)
    except OSError:
        pass

    # a simple page that says hello
    @app.route('/hello')
    def hello():
        return 'Hello, World!'

    return app